An idealized study of African easterly waves. III: More realistic basic states
An idealized study of African easterly waves. III: More realistic basic states
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非洲东波的理想化研究。
DOI：
10.1002/qj.49712152706
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发表时间：
1995
影响因子：
8.9
通讯作者：
C. Thorncroft
中科院分区：
文献类型：
--
作者：
C. Thorncroft
本文研究了三种不同东风急流上不稳定东风波的性质。在急流的赤道方向一侧具有最强水平平均切变的“切变急流”的特征是在急流最大值的赤道方向具有最大振幅的线性不稳定波，并且由与正水平动量通量相关的正压能量转换主导。剪切射流的正常模式在表面处具有非常小的振幅。
具有较低平均静态稳定度的“沙漠急流”更好地代表了急流向极侧的大气条件，其特征在于在表面具有最大振幅的线性不稳定波。尽管正压能量转换仍然较大，但这些模态的斜压能量转换比切变急流的大得多。与垂直动量通量有关的垂直雷诺应力项约占正压能量转换值的25%。
同时还研究了具有增强的向赤道水平切变和低静稳定度的“组合急流”。与该射流相关联的线性简正模式的特征在于射流水平和表面处的显著振幅。正压能量转换到斜压能量转换的比率介于与切变急流和沙漠急流上的模态相关的比率之间。
研究了急流上3000公里波长东风波的非线性行为。所有生命周期的特征是从初始正压能量转换占主导地位的斜压能量转换占主导地位的过渡。剪切喷流生命周期的非线性部分由喷流的极向侧上的负水平动量通量和温度梯度区域中的表面处的最大振幅主导。在不同意的意见，只有非常弱的非线性正水平动量通量存在于赤道一侧的喷流。正常模态的非线性沙漠急流生命周期继续以低层最大振幅为特征，并更快地由斜压能量转换主导。非线性组合喷流生命周期以类似于沙漠喷流生命周期的方式演变，但是具有更显著的喷流水平幅度。
研究了相对涡度在生命周期中的天气演变。一个单一的表面相对涡度最大值，与纬向表面温度梯度已被确定在所有理想化的积分检查，包括那些与潜热加热，在不同意的意见，这表明两个最大值的南北射流。然而，在850毫巴左右，确实存在两个正涡度中心;一个弱的赤道方向的一个与急流层的发展有关，一个较强的极地方向的一个与低层温度梯度有关。
剪切的生命周期重新检查一个简单的潮湿参数化。斜压能量转换增加，但正压能量转换几乎不受潜热加热的影响，急流层结构几乎不变。表面涡度最大值增加。水汽供应和急流之间的关系在确定地面发展的性质和上升模式以及斜压能量转换的大小方面是很重要的。
有人建议，观测和非线性赤道结构之间的差异，在这里确定的，可以减少一个更准确的表示非绝热过程。
The nature of unstable easterly waves growing on three different easterly jets has been examined. the ‘shearjet’ with strongest horizontal mean shear on the equatorward side of the jet is characterized by linearly unstable waves with maximum amplitude equatorward of the jet maximum and are dominated by barotropic energy conversions associated with positive horizontal momentum fluxes. the normal modes of the shear-jet have very little amplitude at the surface.
The ‘desert-jet’ with lower mean static stability better represents the atmospheric conditions on the poleward side of the jet, and is characterized by linearly unstable waves with maximum amplitude at the surface. the modes have much larger baroclinic energy conversions than those of the shear-jet, although barotropic energy conversions are still larger. About 25% of the value of the barotropic energy conversions is attributable to the vertical Reynolds stress term associated with vertical momentum fluxes.
The ‘combined-jet’ with both enhanced equatorward horizontal shear and low static stability combined, has also been examined. the linear normal modes associated with this jet are characterized by significant amplitudes at both the jet-level and at the surface. the ratio of barotropic to baroclinic energy conversions is between that associated with the modes growing on the shear- and desert-jets.
The nonlinear behaviour of 3000 km wavelength easterly waves growing on the jets has been examined. All life-cycles are characterized by a transition from initial barotropic energy conversions dominance to baroclinic energy conversions dominance. the nonlinear part of the shear-jet life-cycle is dominated by negative horizontal momentum fluxes on the poleward side of the jet and maximum amplitude at the surface in the region of the temperature gradients. In disagreement with observations only very weak nonlinear positive horizontal momentum fluxes exist on the equatorward side of the jet. the nonlinear desert-jet life-cycle as for the normal mode continues to be characterized by maximum amplitudes at low-levels and becomes dominated by baroclinic energy conversions sooner. the nonlinear combined-jet life-cycle evolves in a similar manner to the desert-jet life-cycle but with more significant jet-level amplitudes.
The synoptic evolution of the relative vorticity in the life-cycles is examined. A single surface relative vorticity maximum, associated with the meridional surface temperature gradient has been identified in all idealized integrations examined, including those with latent heating, in disagreement with observations which indicate two maxima north and south of the jet. At about 850 mb however, two positive vorticity centres do exist; a weak equatorward one associated with the developments at the jet level and a stronger poleward one associated with the low-level temperature gradients.
The shear life-cycle is re-examined with a simple moist parametrization included. Baroclinic energy conversions increase but barotropic energy conversions are almost unaffected by the latent heating and the jet-level structure almost unchanged. the surface vorticity maxima is increased. the relationship between moisture availability and the jet is shown to be important in determining the nature of surface developments and the ascent pattern as well as the magnitude of the baroclinic energy conversions.
It is suggested that differences between observations and the nonlinear equatorward structures identified here, may be reduced with a more accurate representation of diabatic processes.